If you've never replaced the O2 sensors and your vehicle has over 75K miles, I'd say it's safe to go ahead and replace them anyway.
Are you getting any codes?
My Mazda3 has around 130,000 miles. So I guess I could swap out the O2 sensors. Any recommendation on brand? NTK or Denso?
Yes I did get a fault code about a week ago. It was a P2187 fault code and I had a P2187 pending code as well. P2187 is "system too lean at idle (bank 1)".
I checked for vacuum leaks but the only thing that looked suspect to me was my MAF o-ring was a bit stretched out. I've ordered a new one and will get it installed once I get it in hand.